On the origin of flux dependence in pH-modified skim milk filtration

Abstract – The aim of this study was to contribute to identify the physico-chemical origin of flux variations, namely the limiting flux (maximum) and the critical flux (sustainable), in ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) of skim milks modified by addition of HCl or NaOH. Both the limiting and the critical fluxes varied in a nonpredictable way with pH but with close similar trends in UF, NF and RO, highlighting the leading role of the fluid behaviour. Physicochemical characteristics of caseins, such as size and electrophoretic mobility, were measured in the pH range 1.9–11.5 and were correlated to fluxes in UF and to a less extent in NF and RO. Available data on the electrophoretic mobility of α-lactalbumin and β-lactoglobulin allowed to suggest that serum proteins would also participate in the flux variations with a possible specific impact of α-lactalbumin as an internal foulant in UF. The role of calcium and inorganic phosphate over the wide pH range was discussed by taking calculated salt equilibrium of milk as a function of acidic pH using a new software and new additional analyses in alkaline media. Results underlined the determining role of Ca2+ in the inorganic irreversible fouling of organic membranes allowing proposal of a simplified cleaning protocol over a wide pH range between 6.7 and 11.5